The present invention relates to transmission devices employed in particular in motor vehicles between the engine unit and front driving wheels and of the type comprising two constant-speed joints, at least one of which is slidable, interconnected by a transmission shaft.
In such an arrangement, the intermediate transmission shaft is connected at each end with an element of the adjacent joint. Thus, if the two constant-speed joints are of the tripod type, one being fixed and the other slidable, the transmission shaft is connected at one end to the tulip element of the fixed joint and is connected at its other end to the tripod element of the slidable joint.
There are used as intermediate transmission shafts solid shafts which have the drawback of being heavy and, owing to their weight and their resilience, of transmitting and even amplifying the various vibrations produced by the engine unit.
Tubular transmission shafts of mild steel have also been used which have at each end inner splines or serrations and connected to the elements of the adjacent joints by the shrinkage of their serrated end regions onto the externally splined or serrated shaft sections and extending said elements. According to the type of steel employed, the outside diameter and the thickness of these tubular shafts are relatively great and their weight is higher than that of a solid bar capable of transmitting the same torque. Further, such a tubular transmission shaft does not solve in a satisfactory manner the problem of the transmission or amplification of certain vibrations.
There is known from French Pat. No. FR-A-2 345 522 an arrangement in which a tubular transmission shaft is welded at its end to a universal joint yoke. This welded assembly is then heat treated by a treatment comprising heating followed by quenching and tempering, the purpose of which is to improve the the structural resistance characteristics of the parts and to remove the residual stresses resulting from the welding.
An assembly treated in this way is unsuitable in the particular application envisaged where the transmission shaft carries, at at least one of its ends, a constant-speed joint element defining raceways for rollers or balls, which raceways being therefore called upon to support contact pressures or Hertzian pressure of high value, for example of the order of 400 kg/mm.sup.2. Usually, this element of the joint is therefore subjected, before it is mounted on the transmission shaft, to a thermochemical treatment adapted to impart thereto the desired properties of surface hardness. Moreover, it is known that this type of treatment can, under certain conditions, have a negative effect on the performance and strength of the parts subjected thereto and it has even been necessary, in order to overcome this drawback, to provide untreated regions, ie. to mask certain parts of these parts so as to prevent them from coming into contact with the treatment gas.